Archived herbage samples taken between the mid-1800s and the present day from the Park Grass permanent grassland experiment in the UK were analysed for PCDD/Fs. The concentrations of ∑P(4–8)CDD/Fs ranged between 15 (in 2003–2004) and 320 pg/g (in 1963) and the ∑TEQ ranged between 0.11 (2003–2004) and 2.4 pg/g in (1903). The underlying trend from the mid-1970s to the present is of a decline by about factor of 10, indicating a general reduction in the air concentration/deposition fluxes. The homologue pattern in samples from the first half of the 20th century was characterised by the lower (mono- to tri-) PCDFs, indicating the dominance of domestic wood/coal burning on the ∑P(1–8)CDD/Fs signature. The second half of the 20th century saw a substantial decline in domestic wood/coal burning for space heating in the UK, but also the ‘rise and fall’ in the production/use of chloroaromatic compounds—notably pentachlorophenol (PCP). The isomer/homologue patterns for the 1960–2004 samples have a much lower contribution from the lower PCDFs and large contributions from the hepta and octa-CDDs. The possibility that these are related to PCP inputs via different routes is discussed. The UK—in line with other countries—has had a policy to reduce the environmental sources and the emissions of PCDD/Fs, by the introduction of new combustion control technologies and emissions standards. However, these were not introduced to specifically address PCDD/F emissions until the 1990s. The declines in PCDD/F levels in these samples: (a) pre-date the introduction of emission control measures on incinerators and other combustion sources in the UK; (b) appear to have been largely unaffected by them.